While it wasn’t apparent at the scene Friday, city officials blame Combination Pond for heating up Moon Brook, and have challenged the state’s recent stormwater regulations.

Vermont’s Environmental Court will be called on to decide whether the sun or the rain keeps trout out of Moon Brook.

Rutland has appealed the new state stormwater permit as it applies to Moon Brook, arguing that the regulation would require the city and private property owners to spend millions of dollars without addressing the root problem of the impaired watershed.

At the heart of the dispute is the question of just what is impairing Moon Brook. Impairment is determined by measuring the biota (small-scale plant and animal life) necessary for the brook to function as a trout habitat.

The state holds that storm runoff and associated pollution and sediment entering the brook is the problem, and calls for the creation of ponds, holding tanks and other infrastructure to keep the stormwater out.

The city, on the other hand, notes that temperatures in the brook are regularly too high for trout to survive, and Mayor Christopher Louras pointed a finger at Piedmont Pond and Combination Pond, where water sits and grows warm before flowing through the rest of Moon Brook, as the culprits.

Louras said the city has an estimate putting the cost of various stormwater mitigation work at $25 million to $40 million and that the city would likely need to take on a full-time stormwater management staff.

“While the city wants to see water quality standards met, we can’t afford to address both the temperature issue and the stormwater issue,” he said. “We could reduce the stormwater runoff to zero and the standards still might not be met. It would be a monumental waste of private and public money.”

Peter LaFlamme, director of the Department of Environmental Conservation’s Watershed Management Division, said that while there were clearly elevated temperatures in the water coming out of Combination Pond, it was cooler downstream and that, in all likeliness, the brook’s impairment was due to a combination of temperature and runoff.

“We’ve been back and forth with the city over a number of years on this and we disagree with the city’s position,” he said.

LaFlamme also pointed out that stormwater issues can lead to temperature issues.

“You have your classic August day, hot and baked pavement, and then you get a storm,” he said. “The runoff can heat the brook.”

Anthony Iarrapino, a staff attorney focusing on water quality issues at the Conservation Law Foundation, said the state’s track record was such that if the state disagreed with a town on an environmental issue, it likely means the state has a sound position.

“In my experience, (the Vermont Agency of Natural Resources) is overly conservative in deferring to municipalities,” he said. “It runs very counter to the experience of water quality advocates that ANR would be overreaching when it comes to a municipality as important as Rutland is in the state.”

While the city was the sole municipality to have filed an appeal as of late Friday afternoon — the deadline was 4:30 p.m. Friday — Louras said he intended to reach out to Rutland Town about having them join in the appeal.